Field of the Invention
The present invention relates to a process and plant for the heat-treatment of metals in a protective atmosphere, e.g. annealing, normalization, pre-tempra heating.
In these types of process, the atmosphere used in the furnace must be neutral, not carburizing or decarburizing, to avoid modification of the surface composition of the treated metal; the atmosphere could be slightly reductive to eliminate any oxygen which enters the heat treatment furnace.
Traditional heat treatment processes are known in which the protective atmosphere is produced by an exothermic generator in which a combustion reaction with a hydrocarbon takes place in a shortage of air, with comburant:fuel ratios (e.g. for methane) from 1:6 to 1:9. This process has the disadvantage of producing large quantities of CO.sub.2 and H.sub.2 O which must be at least in part removed from the mixture.
There are also known processes which use an endothermic generator to obtain the desired atmosphere from a mixture of air and hydrocarbons. The comburant:fuel ratio for reaction is 2:1 when methane is used. USSR application No. 523144, filed Mar. 27, 1975, discloses a method of preparing a protective atmosphere for metal treatment plants according to which commercial nitrogen, containing O.sub.2 impurities, is mixed with natural gas in the amount of 2.0-2.5 volumes of of the oxygen present in the nitrogen. The mixture is fed to a reactor containing a Nickel catalyst, converted and fed to the furnace of the plant.
European Patent Application N.degree.0482992, filed Oct. 22, 1991 in the name of AIR LIQUIDE, describes a process for obtaining a protective atmosphere with a low content of reducing agents by passing nitrogen with O.sub.2 content of between 1% and 7% through a catalytic reactor provided with a precious metal catalyst at a temperature of between 400.degree. C. and 900.degree. C. On the one hand, this process has the advantage of producing an atmosphere with H.sub.2 and CO contents in the same order as those of the exothermic reaction, but with low CO.sub.2 and water contents; on the other, it requires the use of fairly expensive catalysts and is poorly suited to the treatment of high- to medium-carbon steels.
This document also mentions the possibility of operating at high temperatures with a Nickel-based catalyst, as disclosed by the USSR application above discussed, but judges such a process unsuitable for industrial production and advises against its use.